This invention relates generally to an electrophotography, and more particularly, to an electrophotographic apparatus comprising a transfer roller for transferring a toner image onto a record paper.
Heretofore there have been developed various kinds of electrophotographic apparatuses in which plain papers are used as record papers. In such plain paper copying (PPC) machines an electrostatic charge latent image corresponding to a document image is formed on a photosensitive member, the latent image is developed with toners and the toned image is transferred onto a plain paper so as to obtain a copy after fixing the toner image.
There have been proposed various kinds of multiple copying machines in which a plurality of copies of a document are formed from the same and single electrostatic charge latent image once formed on a photosensitive body by repeatedly effecting developing and transferring steps for the same electrostatic charge latent image.
The present invention is particularly useful for such a multiple copying apparatus. In particular the present invention can be advantageously applied to a multiple copying machine in which the toned image is transferred onto a plain paper by means of a transferring roller to which is applied a suitable transferring bias voltage.
In the electrophotographic apparatus comprising the transfer roller attention should be paid to prevent toner particles disposed on the photosensitive member from being adhered or transferred to the transfer roller. That is to say it is necessary to avoid an off-set development of residual toners onto the transfer roller. To this end various measures have been developed. For instance, the transfer roller is urged against the photosensitive body only when the record paper is existent between the transfer roller and the photosensitive body. In this case it is also proposed to cut off the supply of the transferring bias to the transfer roller during the periods when the roller is separated from the photosensitive body. It is further proposed to discharge or erase the electrostatic charge on the photosensitive body as soon as the transfer roller is separated from the body. It is also known that upon the detection of non-existence of the record paper at the transferring section a polarity of the transferring bias voltage is made reversed.
In the known copying machine comprising the transfer roller the roller is always in contact with the photosensitive body under a given pressure as long as the record paper exists between the transfer roller and the photosensitive body. Therefore when a paper jam occurs at the transfer section, the jammed paper is retained between the transfer roller and the photosensitive body and thus it is quite difficult to remove the jammed paper without damaging or injuring the very expensive photosensitive body. That is to say since the transfer roller clamps the jammed paper between the roller and photosensitive member under a substantial pressure the jammed paper has to be pulled forcedly against the clamping force of the roller. Therefore the jammed paper rubs strongly the photosensitive body and roller and as the result the photosensitive body and roller are injured seriously.
When the transfer roller remains in contact with the photosensitive member even after the jammed paper has been removed, the residual part of the toned image on the photosensitive member is transferred to the transfer roller upon starting again the duplicating operation. In order to avoid such inconvenience the polarity of the transfer bias voltage may be reversed after removing the jammed paper. But in this case the toner image of high density which has been transferred neither to the transfer roller nor to the record paper has to be cleaned by a cleaning device such as a rotating cleaning brush. Therefore the cleaning device is liable to be overloaded and could not sufficiently brush off the residual toners on the photosensitive body, and thus the photosensitive body still retains a substantial amount of toner particles. When the duplication is started again, a uniform charging has to be effected through the residual toners and the uniform charging level could not be obtained. Therefore decrease and fluctuation in a copy density and overdevelopment might occur and an image quality of the duplicated copy is extremely deteriorated.
The uniform charging level on the photosensitive member is the most important factor in the above mentioned multiple copying machine, because it determines an amount of charge of the latent image and then this amount determines the maximum number of times of repeatedly effected duplication, i.e. the maximum obtainable copy number. Therefore if the charging level deviates from a given value due to the residual toners on the photosensitive member, the image quality of copies gradually decreases in accordance with the successive duplications and thus an ability of making the multiple copies from the single latent image is extremely limited. The above mentioned problem may also occur in an electrophotographic apparatus comprising a transfer endless belt instead of the transfer roller.